add txt file format

[nobodyiam]

Fixes #1953

[codecov-io]

Codecov Report

Merging #2181 into master will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@             Coverage Diff              @@
##             master    #2181      +/-   ##
============================================
- Coverage     49.95%   49.92%   -0.03%     
- Complexity     1978     1979       +1     
============================================
  Files           401      402       +1     
  Lines         12395    12400       +5     
  Branches       1273     1273              
============================================
- Hits           6192     6191       -1     
- Misses         5761     5765       +4     
- Partials        442      444       +2

Impacted Files	Coverage Δ	Complexity Δ
...rip/framework/apollo/spi/DefaultConfigFactory.java	91.42% <0%> (-2.69%)	18 <0> (ø)
.../framework/apollo/core/enums/ConfigFileFormat.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...trip/framework/apollo/internals/TxtConfigFile.java	100% <100%> (ø)	2 <2> (?)
...mework/apollo/portal/component/PortalSettings.java	65.07% <0%> (-4.77%)	5% <0%> (ø)
.../apollo/internals/RemoteConfigLongPollService.java	79.62% <0%> (-0.62%)	27% <0%> (-1%)

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[coveralls]

Coverage decreased (-0.01%) to 53.508% when pulling f21610d on nobodyiam:add-txt-format into dd0a567 on ctripcorp:master.